reflect: add MapIter.Reset

This allows callers to do (amortized) allocation-free iteration
over many maps.

Fixes #46293

Change-Id: I3aa6134dd00da35b508bd1e3b487332a871a3673
Reviewed-on: https://go-review.googlesource.com/c/go/+/321891


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>